1. Field of the Invention
The present invention is in the field of wireless power transfer. The present invention is further in the field of radio-frequency power transfer. The present invention is further in the field of ultrasound power transfer. The present invention is further in the field of wireless power transfer within mobile telephones and tablets. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into larger integrated circuits.
2. Brief Description of Related Art
With the proliferation of portable devices there is a growing demand for battery charging means that are efficient and user friendly. One of the most recent methods of charging the portable communication devices, as the likes of cellular telephones and tablets, is the utilization of wireless power transfer, that is without the use of wired charger or wall warts.
Nowadays there are typically two ways to achieve the charging by Radio-Frequency (RF) means: the first one is inductive coupling and the second one is RF resonating near field energy transfer. Inductive coupling is related to the way that power is transferred in a transformer. The magnetic field generated between the primary and the secondary windings of the transformer generates current in the secondary that can be rectified and processed for charging the battery. This first method is quite efficient but it requires that the primary and secondary windings be located very close to each other. In addition any foreign object made of metallic material can adversely interfere the energy transfer operation causing the efficiency to drop significantly while overheating the foreign object.
In the case of Radio Frequency resonating near field energy transfer the operation is similar to the common data radio communication between two points, with the difference that the energy radiated and, most importantly, the energy captured by the receiver is several orders of magnitude higher than the one used for data communication. A resonating circuit at the receiver side is tuned to the same frequency of the transmitter in order to capture most of the radiated energy at that frequency. However also in this case the magnetic field is the main media for energy transfer and generally the transmitter and the receiver have to be in close proximity of each other.
Both methods have been studied and employed to charge the battery of a cellular telephone, of a smart-phone or of a tablet. In these systems the power transmitter is located somewhere in proximity of the portable device and the wireless power receiver is positioned inside the device itself. For example Cook et al (US 2012/0262004) describes a typical charging system where the charger is clearly external to the portable communication device. In the case of smart-phones and tablets, two consortia of several companies were created to define the standards of the wireless power transfer parameters.